A Manassas man involved in a head-on collision yesterday morning on Rte. 234 has become the 15th highway fatality this year in Prince William County, which ranked third in Virginia in traffic deaths last year.

Thomas W. Buell, 33, of the 12000 block of Spring Drive, died at Prince William Hospital, where he was taken after his van collided with a dump truck driven by James Avent, police said. Avent, who lives in Northeast Washington, was hospitalized in serious condition.

With a population of 199,000, Prince William County, a rapidly growing bedroom community 25 miles southwest of Washington, recorded more traffic fatalities last year -- 33 -- than many Virginia localities with larger populations, among them Norfolk and Richmond.

Police and others familiar with traffic and road conditions in Prince William cite the county's staggering growth and a road network inadequate to the task of handling the ever-increasing number of vehicles in explaining the high number of road fatalities.

Last year Prince William trailed only Fairfax County, with 84 traffic fatalities, and Virginia Beach, with 51, according to statistics compiled by the Virginia Department of Motor Vehicles. Both jurisdictions have much larger populations than Prince William's: Fairfax County has about 700,000 residents; Virginia Beach has about 362,000.

Norfolk, with a population of 282,900, had 31 traffic deaths in 1986; Richmond, with 219,000 residents, had 19.

Prince William also ranked among the top five Virginia localities in traffic fatalities in 1985, with 23 deaths.

Police and others in the county suggest that the rising numbers are a result of the rapid urbanization of the once-rural area. Prince William's population has nearly quadrupled since 1960 and is expected to top 250,000 by the year 2000.

"Whenever the population of a locality increases, the number of crashes increases proportionately," said Richard Adams, program director at the Department of Motor Vehicles.

Overcrowding on county roads is one of the main factors contributing to the number of traffic deaths, said Maj. J.K. Sullivan of the Prince William County Police Department. The situation is worsened by the fact that the overcrowding is occurring on roads not designed to handle a high volume of traffic, Sullivan added, noting that much of Rte. 234 and Davis Ford Road, the county's primary north-south links, is two lanes. "In some places there is nowhere to go" to avoid an accident, Sullivan said.

Of the 15 highway fatalities in the county this year, all but one resulted from accidents on or near Davis Ford Road or Rte. 234. Last year Davis Ford Road recorded a traffic count ranging from 25,000 to 11,435 vehicles a day, measured at various locations, according to the Virginia Department of Transportation. Rte. 234 recorded from 27,000 to 4,750 vehicles daily last year.

In addition to increased traffic, speed is a contributing factor in fatal traffic accidents, Adams said. Primary roads, which are prevalent in Prince William, tend to have higher speed limits and thus are more likely to have more serious accidents, Adams said. Both Davis Ford Road and Rte. 234 have sections where the speed limit is 55 mph. In addition, two interstates cut through the county.

Nationwide, there were an estimated 46,000 highway fatalities in 1986, an increase of about 5 percent over 1985, according to statistics released this week by the U.S. Department of Transportation.

About 1,000 of those deaths occurred on Virginia highways, Adams said. Maryland had about 790 fatalities last year, according to state police, with 109 in Prince George's County (population 678,000) and 31 in Howard County (population 157,000). The District (population 600,000) recorded 46 traffic deaths last year, police said.

About 66 percent of Virginia's traffic fatalities result from accidents in which alcohol is a factor, Adams said; in Prince William, the figure is less than 50 percent, according to Sullivan.

"People need to be full-time drivers rather than thinking about the meeting or what they are going to do when they get home," he said.